1. Field of the Invention
The present invention relates to wafer level fabrication of high value inductors on semiconductor integrated circuits, and more particularly, to the optimization of power inductor arrays on integrated circuits for switching regulator applications.
2. Background of the Invention
Inductors are commonly used in the electronics industry for storing magnetic energy. Providing an electric current though a metal conductor, such as a metal plate or bar, typically creates an inductor. The current passing though the metal conductor creates a magnet field or flux around the conductor. The amount of inductance is measured in terms of Henries. In the semiconductor industry, it is known to form inductors on integrated circuits. The inductors are typically created by fabricating what is commonly called an “air coil” inductor on the chip. The air coil inductor is usually either aluminum or some other metal patterned in a helical, toroidal or a “watch spring” coil shape. By applying a current through the inductor, the magnetic flux is created.
Inductors are used on chips for a number of applications. Perhaps the most common application is DC-to-DC switching regulators. In many situations, however, on chip inductors do not generate enough flux or energy for a particular application. When this occurs, very often an off-chip discrete inductor is used.
There are a number of problems in using off-chip inductors. Foremost, they tend to be expensive. With advances in semiconductor process technology, millions upon millions of transistors can be fabricated onto a single chip. With all these transistors, designers have been able to cram a tremendous amount of functionality onto a single chip and an entire system on just one or a handful of chips. Providing an off-chip inductor can therefore be relatively expensive compared to the overall cost of the system. Off-chip inductors can also be problematic in situations where space is at a premium. In a cell phone or personal digital assistant (PDA) for example, it may be difficult to squeeze a discrete inductor into a compact package. As a result, the consumer product may not be as small or compact as desired.
An apparatus and method for wafer level fabrication of high value inductors directly on top of semiconductor integrated circuits is therefore needed.